1. Technical Field
This invention concerns semiconducting nonstoichiometric oxide materials to be used as oxygen sensors to measure oxygen partial pressures in hot gases.
2. Description of the Related Art
Oxygen sensors based on the measurement of electrical resistivity of semiconducting oxides have been proposed. For example D. E. Williams and P. McGeehin in their article "Solid State Gas Sensors and Monitors" published in Electrochemistry, Vol 9, p 246-290 (1984) have discussed the characteristics of this and other types of sensors. Such devices are considered as alternatives to Nernst sensors which are currently used for measuring oxygen partial pressure in combustion products of boilers and car engines. An International patent application No. PCT/AU84/00013 describes one such Nernst sensor.
In the Nernst sensor, a solid electrolyte membrane wirh good oxygen ion conductivity and negligible electronic conductivity separates a reference gas from the test gas. When two electrodes reversible to the O.sub.2 /O.sup.2- redox equilibrium are placed in contact with the opposite faces of the membrane, an emf is established which is proportional to the oxygen concentration difference across the membrane. To avoid mixing of reference gas with the test gas, long impervious tubes of the solid electrolyte need to be used. Alternatively, hermetic seals are formed between a solid electrolyte membrane and a more robust tube of a ceramic or a metal.
On the other hand, the construction of a sensor based on measurement of electrical resistivity of a semiconducting nonstoichiometric oxide material is very simple and no hermetic seals are required. In its simplest form, the nonstoichiometric semiconducting oxide sensor consists of a porous disc of the material or a thin layer of the material painted on an insulator substrate such as alumina and two separate electrical contacts to measure the resistance. The principle of operation of a resistance measuring oxygen sensor is that the resistance of a semiconducting nonstoichiometric oxide material is represented by the expression: EQU R.sub.o .varies.exponential (E/RT) (pO.sub.2).sup.n
where:
R.sub.o =resistance of the material, PA1 T=the absolute temperature, PA1 E=the activation energy for conduction, PA1 (pO.sub.2)=the oxygen partial pressure, and PA1 R=the gas constant. PA1 (i) The construction of the sensor is extremely simple and the sensor can be prepared in any shape and size; PA1 (ii) The sensors are very adaptable to miniaturization which enables accurate and uniform temperature control; PA1 (iii) The current voltage relationship is linear and the sensitivity of the sensor can be increased by simply increasing the current; PA1 (iv) Response time can be increased by construction of thin and/or porous films of the sensing material. PA1 Irreproducible value of n arising from material preparation, sensor geometry and other factors; PA1 (ii) Slow oxidation kinetics at the gas/oxide interface; PA1 (iii) Lack of linear relationship between resistance and oxygen partial pressure over the useful oxygen concentration range (0.1-10% O.sub.2); PA1 (iv) High value of activation energy or low value of n; and PA1 (v) Change in the surface conductivity due to adsorption of impurities from flue or other gases being measured. PA1 (a) determining the calibration constant R.sub.1, being the resistance at the said certain temperature of a sensor according to the invention in an atmosphere having a known oxygen partial pressure p.sup.c O.sub.2 ; PA1 (b) determining the resistance R.sub.O at the said certain temperature of the sensor in the hot gas whose oxygen partial pressure p.sup.u O.sub.2 is to be measured; and PA1 (c) using the formula EQU p.sup.u O.sub.2 (atm)=p.sup.c O.sub.2 (R.sub.0 /R.sub.1).sup.1/n PA1 to calculate the oxygen partial pressure in the hot gas, where n has a predetermined value for the sensing material of the sensor at the said certain temperature and is a measure of the defect chemistry of the sensing material. PA1 (a) determining the calibration constant R.sub.1 being the resistance at a known temperature of a sensor according to the invention in an atmosphere having a known oxygen partial pressure p.sup.c O.sub.2; PA1 (b) determining the resistance R.sub.0 of the sensor in the hot gas whose oxygen partial pressure p.sup.u O.sub.2 is to be measured; PA1 (c) determining the temperature T of the hot gas at which the resistance R.sub.0 is determined and applying a correction to the calibration constant R.sub.1 for variation of temperature T from said known temperature making use of the known relationship EQU R.sub.1 .varies.exponential (E/RT) PA1 where PA1 (d) determining the modified value (n') of n at the temperature of the sensor from a previously known relationship between n and temperature; and PA1 (e) using the formula EQU p.sup.u O.sub.2 (atm)=p.sup.c O.sub.2 (R.sub.0 /R.sub.1.sup.c).sup.1/n' PA1 to calculate the oxygen partial pressure in the hot gas, where R.sub.1.sup.c is the corrected calibration constant.